Determination of sulfate using ferric ion-selective electrode

ABSTRACT

Sulfate ion concentration in aqueous solution is determined by initially adding a known concentration of ferric ion to the solution, and adjusting the pH of the solution to a suitable value, whereby ferric ion is complexed by the sulfate ion. The activity of the remaining, uncomplexed ferric ion is then measured by means of a ferric ion-selective electrode comprising a glass of the formula FenGexSbySez, where n is about 1.5 to 3.0, x is about 27 to 29, y is about 11 to 13 and z is about 59 to 61.

United States Patent [191 Jasinski et al.

1 July 30, 1.974

[ DETERMINATION OF SULFATE USING FERRIC ION-SELECTIVE ELECTRODE [75]Inventors: Raymond J. Jasinski; Isaac Trachtenberg, both of Dallas, Tex.

[73] Assignee: The United States of America as represented by theSecretary of the Interior, Washington, DC.

[22] Filed: Apr. 12, 1973 1211 Appl. No.: 350,444

[52] US. Cl. 204/1 T, 204/195 G, 204/195 M [51] Int. Cl. G01n 27/46 [58]Field of Search 204/1 T, 195 G, 195 M [56] References Cited 1 UNITEDSTATES PATENTS l/l973 l/1973 Johnson et a1 204/195 G OTHER PUBLICATIONSNational Bureau of Standards Special Publication 314,

Saunders 204/195 M Nov., 1969, PP- 367-370, 430.

Orion Application Bulletin No. 11, 1969, pp. 1 & 2.

Primary Examiner- T. Tung Attorney, Agent, or Firm-William S. Brown;Frank A. Lukasik [5 7] ABSTRACT Sulfate ion concentration in aqueoussolution is determined by initially adding a known concentration offerric ion to the solution, andadjusting the pH of the solution to asuitable value, whereby ferric ion is complexed by the sulfate ion. Theactivity of the remaining, uncomplexed ferric ion is then measured bymeans of a ferric ion-selective electrode comprising a glass of theformula Fe Ge Sb Se where n is about 1.5 to 3.0, x is about 27 to 29, yis about 11 to 13 and z is about 59 to 61.

5 Claims, 2 Drawing Figures DETERMINATION OF SULFATE USING FERRICION-SELECTIVE ELECTRODE Detection and measurement of the concentrationof various ions in aqueous solution by means of ion selective electrodesfinds utility in a variety of environmental and industrial applications.Analytical potentiometry, and ion selective electrodes or sensorstherefor, can be employed for control of water quality, to obtainoptimum yields in chemical processes, etc. Prior art sensors for copper,iron and sulfate ions are disclosed in copending application Ser. No.335,397, filed Feb. 23, 1973, in US. Pat. No. 3,709,813 and in US. Pat.No. 3,709,811, respectively. In the latter, a variety of prior artprocess for measurement of sulfate ion concentration are disclosed.However, these processes have generally been deficient in variousproperties such as sensitivity, selectivity and reliability.

It has now been found, according to the process of the invention, thatsulfate ion may be detected or measured in aqueous solution by means ofan indirect process that is free of most of the prior art limitations.This process involves (1) addition to the test solution, i.e., thesolution whose sulfate ion content is to be measured, of a knownconcentration of ferric ion, (2) adjustment of the pH of the testsolution to a value of about 1.5 to 3, preferably about 2, and (3)measurement of the resulting activity of the ferric ion in the solutionby means of a ferric ion-selective sensor, as described below.

The principle of operation on which the process of the invention isbased depends on (1) the response of the ferric ion-selective sensor tohydrated ferric ion only and (2) the complex-ion forming reactions ofsulfate ion with ferric ion. The ferric ion selective sensor willrespond only to ferric ion in the hydrated form (hereafter referred toas Fe). It will not respond to ferric ion in the sulfate-complexedstate, even though the valence of the iron itself is still trivalent.

The chemical environment of the test solution is so chosen that theferric iron-sulfate complexes are sufficiently strong to remove some ofthe ferric iron from the hydrated form, but not so strong as to purgethe solution entirely of the hydrated form. The quantity of Fe +3present, and thus sensed by the ferric ion-selective sensor, is afunction of the total ferric iron present in the test solution, pH ofthe test solution and total sulfate ion concentration in the testsolution. Therefore, fixing two of these variables, i.e., pH and totalferric iron, and measuring Fe defines the total sulfate ionconcentration.

The process of the invention may be used to measure sulfate ionconcentrations in solutions containing sulfate in concentrations ofabout 0.005 moles per liter to saturation. Suitable concentrations offerric iron in the test solution will generally range from about 10 to0.1 moles per liter. Optimum values of the concentration of ferric iron,as well as pH, may vary considerably depending on the range of sulfateion concentration, other ionic species in the test solution, specificferric ion-selective sensor employed, etc., and are best determinedempirically. Other ionic species that may be present in the testsolution without interfering with the measurement of sulfate ion includeCl, N0 C Br, 1, Zn, Pb and Na".

If the test solution initially contains a known concentration of ferricion within the above limitations, addition of further ferric iron maynot be necessary. If, however, addition of further ferric iron isnecessary, it may be added in the form of a soluble inorganic ferricsalt such as ferric chloride or ferric nitrate. Addition may be in theform of the salt or an aqueous solution thereof, provided only that thefinal concentration of ferric iron (combined Fe and sulfate-complexedferric ion) in the test solution is known, and is within theabove-mentioned range.

Adjustment of the pH of the test solution, if necessary, may be made byaddition of conventional reagents such as hydrochloric or nitric acids,or bases such as sodium hydroxide or ammonia.

The ferric ion-selective sensor used in the process of the invention isdisclosed in above-mentioned US. Pat. No. 3,709,813. It consistsessentially of a glass membrane of a particular composition, one side ofwhich contacts a reference solution and the other side of which contactsthe test solution. The glass membrane consists of an iron-doped glasscomposition having the formula Fe Ge Sb Se where n is about 1.5 to 3.0,x is about 27 to 29, y is about 11 to 13 and z is about 59 to 61. Thiscomposition is prepared by a conventional glass making technique, suchas that described in Pat. No. 3,709,813. This consists of compounding amixture of the components of the composition in the proportions desiredin the glass product. This mixture is placed in a quartz ampule, whichis then evacuated to about 10' to 10' mm of Hg, sealed and placed in arocking furnace. The mixture is then slowly heated to a temperaturesufficient to ensure that the reactants are in a liquid state. Atemperature of about 900 to l00OC is usually sufficient. A gentlerocking motion is imparted to the furnace to mix and react theconstituents for a period of about 16 to 24 hours. The resulting moltenmass is slowly cooled to about 700C, and rapidly air-quenched to thesolid state. It is then annealed at a temperature of about 275C forabout 2 to 3 hours to remove any strains that may have developed duringthe cooling cycle.

The Fe, Ge, Sb and Se are preferably employed in essentially pure formand in the form of powders of mesh size of about 40 to 200. The ironmay, however, be employed in the form of fine wire or mesh, of adiameter of about 0.007 to 0.012 inch. It may also be added in the formof a compound such as FeSe. As also discussed in Pat. No. 3,709,813, theglass composition may be prepared either by reacting a mixture of allthe ingredients, or by doping the prereacted chalcogenide glass, i.e.,the Ge,,Sb,,Se glass, by addition of the appropriate amount of iron,with a subsequent reaction under the above-described conditions to takethe iron into solution.

The method of the invention will now be described with reference toFIG. 1. The Fe Ge Sb Se glass composition is employed in the form of amembrane (reference l in FIG. 1) consisting of a disc having a thicknessof about 1.0 to 2.5 mm and a diameter of about 5 to 10 mm. Optimumthickness and size of the disc may, however, vary considerably dependingon the specific size and type of apparatus employed, the type ofsolution to be monitored, the reference electrode, etc., and are bestdetermined empirically.

Membrane l is sealed in the bottom of tube 2, which may be of anysuitable inert material, such as Plexiglass (poly-methylmethacrylatepolymer), and is immersed in solution 3, the test solution, i.e., thesolution to be measured or monitored. Reference electrode 4 is mountedwithin tube 2 which is filled with a reference solution-5. A secondreference electrode 6 is immersed in the test solution and referenceelectrodes 4 and 6 are connected to high-impedance voltmeter 7.

Reference electrodes 4 and 6'may be any conventional referenceelectrodes, such as a saturated calomel electrode or a silver-silverchloride electrode, provided only that they are compatible with the testand reference solutions. Reference solution consists of an aqueoussolution of a fixed concentration of ferric ion and an electrolyte. Thissolution suitably consists of a solution of ferric chloride or ferricnitrate in a concentration of about to 10 mole per liter, and anelectrolyte consisting of sodium chloride, sodium nitrate, potassiumchloride or potassium nitrate in a concentration of about 0.1 to 1.0mole per liter. Preferably, the reference solution consists of anaqueous solution of about 10 M ferric chloride or nitrate and about 0.1M sodium or potassium chloride or nitrate.

According to one embodiment of the method of the invention, potentialresponses of the ferric ion-selective electrode, i.e., the electrodecomprising the Fe,,Ge, ,Sb,,Se glass membrane, the ferric ioncontainingreference solution and the reference electrode immersed therein, versusthe reference electrode immersed in the test solution are plotted as afunction of sulfate content of the test solution. Thus, a definiterelationship between potential and sulfate concentration is determined,and this relationship may be used for measuring sulfate concentration inunknown solutions.

A second possible embodiment of the invention involves chemicaltitration for sulfate ion. Ferric iron concentration and pH of the testsolution are again adjusted as described above. The test solution isthen titrated with an aqueous solution of a known concentration of achemical that will remove sulfate ion from solution. Changes inelectrical potential of the test solution are sensed throughout thetitration by means of the ferric ion-selective sensor. The titrantcontinuously removes complexed and uncomplexed sulfate from solution,and in the process ferric iron is released from the sulfate complex toform the hydrated ferric ion (Fe which is sensed by the ferricion-selective electrode. When the sulfate ion is completely removed, theformation of Fe ceases and the solution potential ceases to change. Thisonset of a constant potential defines the end point of the titration.

The preferred titrant is barium chloride, which removes sulfate as abarium sulfate precipitate. Other titrants may, however, be employed,e.g., lead chloride, lead perchorate and barium perchlorate.

The ferric iron-selective electrode, prepared as described above, may beemployed directly in the method of the invention. However, thesensitivity and selectivity of the electrode is generally improved bypolishing and by conventional activation treatment. Furthermore,reactivation of the electrode may be necessary where sensitivity hasdeclined over a period of use. Activation is conveniently accomplishedby initially etching in caustic, e.g., by rinsing or immersing in asolution of alkali such as a 2.0 to 3.0 molar solution of NaOH or KOH,followed by rinsing with an aqueous solution of 0.1 M NaCl containingsufficient HCl to adjust the pH toabout 2 and, finally, equilibrationwith a solution of ferric ion at a pH of about 1.5 to 3.0 for a periodof about 12 to 24 hours. For this purpose, the electrode may be simplyimmersed in a solution of ferric nitrate or chloride of about 0.0008 to0.005 molar concentration for the required time.

The method of the invention is ordinarily practiced at ambientconditions of temperature and pressure and is capable of detecting ppmsulfate ion in aqueous solution. Furthermore, it may be used formeasuring sulfate concentration in media having a high chloride ionconcentration, e.g., sea water, brackish water and industrial wastewater.

The following example will more specifically illustrate the invention.

EXAMPLE A disc 8.0 mm in diameter and 1.5 mm in thickness, andconsisting of Fe Ge sb Se was prepared by the procedure described above.This disc was sealed in a Plexiglas tube 13 cm long and of CD. of 1.5 cmand ID. of 1.3 cm, and was then mechanically polished and activated byetching with 2.5 molar NaOH solution and rinsing with aqueous 0.1 MNaCl-HCl solution (pH 2), and finally exposing it to 0.1 M ferricnitrate solution at a pH of 1.6 for 30 minutes. The tube was filled witha reference solution consisting of 10 M Fe(NO in 0.1 M KNO and was thenimmersed in the test solution, as shown in FIG. 1. The test solutionconsisted of an aqueous solution of 10 M Fe(NO and sulfuric acid in anamount sufficient to provide varying concentrations of sulfate, ion. ThepH of the test solution was maintained at 1.6 by addition of nitric acidor sodium hydroxide as required. Ag/AgCl reference electrodes wereimmersed in the test solution and in the reference solution, and bothwere connected to a high impedance voltmeter.

Results are shown in FIG. 2, which is a plot of the logarithm of theconcentration of sulfate ion in the test solution against the potentialdeveloped between the reference electrodes.

We claim:

1. A method for measuring sulfate ion concentration in an aqueoussolution comprising adding to said solution a known concentration offerric ion sufficient to provide a concentration of about 10 to 10 molesper liter in the solution, adjusting the pH of the solution to about 1.5to 3, and measuring the potential of the resulting solution by means ofa ferric ion-selective electrode comprising an iron-containing glass ofthe formula Fe Ge Sb Se where n is about 1.5 to 3.0, x is about 27 to29, y is about 11 to 13 and z is about 59 to 61.

2. The method of claim 1 in which the ferric iron is initially added inan amount sufficient to provide a concentration of about 10 moles perliter of solution.

3. The method of claim 1 in which the pH of the solution is initiallyadjusted to about 2.

4. The method of claim 1 in which the glass has the formula Fe Ge Sb sewhere n is about 1.5 to 3.0.

5. The method of claim 4 in which n is 2.0.

2. The method of claim 1 in which the ferric iron is initially added inan amount sufficient to provide a concentration of about 10 3 moles perliter of solution.
 3. The method of claim 1 in which the pH of thesolution is initially adjusted to about
 2. 4. The method of claim 1 inwhich the glass has the formula FenGe28Sb12Se60, where n is about 1.5 to3.0.
 5. The method of claim 4 in which n is 2.0.